1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus, such as a microfilm reader printer, which controls a shutter by means of a pulse.
2. Description of the Related Art
Microfilm is a medium which is widely used for recording and storing a broad variety of information. An image forming apparatus such as a microfilm reader printer is used to project the microfilm and record the microfilm information on paper as required.
For example, in the microfilm reader printer, an image on the microfilm is enlarged and displayed as a projection image, on a projection screen by light from a light source. When the image displayed on the projection screen, i.e., the image on the microfilm, is to be copied, the projection light from the light source is radiated on the microfilm, and the radiated light is focused on a photoconductive drum as an image carrier to form an electrostatic latent image. The latent image is developed by a developing unit, transferred onto a copy sheet by a transfer charger, and fixed on the sheet by a fixing unit. Thereafter, the sheet is discharged from the microfilm reader printer.
In an image forming apparatus such as an image reader printer of this type that generally performs reversed development, regions before and after the leading and trailing ends, respectively, of an effective image are unnecessarily developed. In order to prevent this, in an image forming apparatus disclosed in, e.g., Japanese Pat. Application No. 61-109193, a shutter unit is provided for shielding scanning light from a light source as an exposure means, so that only a portion of the photoconductive drum corresponding to the effective image is exposed.
When the scanning light shielding operation by the shutter unit is controlled in accordance with an arbitrary image region, only a desired image region may be copied (trimmed). However, if an inexpensive AC motor is used as a scanning drive means of an image scanning mechanism instead of a DC motor having a high scanning precision, the following problems occur. The rotating speeds of AC motors vary from one motor to another. Therefore, the shutter unit is controlled in accordance with the rotating speed of each AC motor based on a required image scanning distance. However, in this case, the scanning start/end point of the required image does not sometimes correctly match the opening/closing operation of the shutter, with the result that the required image region and the image copied on the sheet may not always match.